In recent years, energy saving (reduction in fuel consumption) has become an important issue in working machines. Moreover, in excavators, it is requested to suppress an engine rotation speed as low as possible in an idling mode or during an excavation operation (normal operation) in a relatively light load.
Here, merely lowering a rated engine rotation speed itself is not possible when traveling performance needs to be ensured. That is, if the rated engine rotation speed is decreased, a maximum pump flow rate decreases to decrease a traveling speed, and a maximum engine output decreases to decrease traveling ability under power-demanding conditions such as on slope land.
Thus, there is a demand for a controller capable of reducing an engine rotation speed during a normal operation state and securing a traveling speed and traveling power.
From such a viewpoint, as a technique of easily realizing low fuel consumption and low noise during an excavation operation while securing traveling performance, an excavator engine controller in which engine rotation speed control means selects isochronous control when a traveling state is detected and selects droop control when the traveling state is not detected so that an engine rotation speed during an operation is lower than the engine rotation speed (rated engine rotation speed) during traveling is disclosed (see Patent Literature 1).
Further, an engine controller is disclosed in which, when the input from travel operation means is detected, a second target rotation speed which is a predetermined target engine rotation speed during traveling is compared with a first target rotation speed which is a minimum engine rotation speed at which a target pump discharge flow rate is realized with maximum pump displacement and the engine rotation speed is controlled by selecting the larger target rotation speed, that is, the smaller engine rotation speed is selected when a machine is not traveling (see Patent Literature 2).
Further, as an engine controller capable of controlling driving of an engine efficiently with low fuel consumption, an engine controller which detects an operation state of a lever with pilot pressure, detects the type and combination of hydraulic actuators operated from the operation state, applies a target engine rotation speed (upper limit) set in advance individually according to the type and combination, and sets an upper limit of an engine rotation speed in a travel-only mode so as to be higher than the lower limit of the engine rotation speed is disclosed (see Patent Literature 3).
Patent Literature 1: Japanese Patent Application Publication No. 2007-255414
Patent Literature 2: Japanese Patent Application Publication No. 2009-074406
Patent Literature 3: Japanese Patent Application Publication No.2011-157931
As described above, although techniques of implementing change of an engine rotation speed between during operations and during traveling have been known in the art, the conventional engine controllers cannot deal with power-demanding traveling conditions such as on slope land or during spin-turning.